Since then, the Bantams have lost nine of their 19 League Two matches, and taken one point from the last 21 available.
They were appointed after losing one of 11 games as caretaker managers, form that took the club from the thick of a relegation battle into a position where they had a realistic chance of the play-offs.
The pair stepped up from under-18 coaches to take over the first team when Stuart McCall was sacked as manager in December, weeks after being given a new contract.
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They were backed in the January transfer market, making nine signings.
Both have been offered jobs to stay at the club. Trueman has already said he intends to stay.
Bradford say they have already made plans to quickly fill the vacancy, and will not seek applicants.
“After saving us from potentially dropping out of the division, we wanted a strong finish to the season to give us momentum, heading into next,” explained chief executive Ryan Sparks.
“This has not transpired and one point from a possible 21, in the end, has ultimately seen us settle for mid-table - which is far away from where this club wants and needs to be.
“We feel, at this point, we need more experience to take us forward into the summer and next season which, for us, is a hugely significant one. We have a crucial few months in front of us, and it is important we are in the best-possible position to be able to prepare for them.”
Citing the pair’s inexperience seems odd as it was not seen as a barrier when the League’s youngest chief executive confirmed them in their jobs “permanently” less than three months ago.
Although a 15th-place finish was undoubtedly disappointing for a former Premier League club, avoiding relegation with seven weeks to spare was a considerable achievement for first-time managers who took over a side one place above the bottom two.
“Their efforts and achievements will be remembered at this club for being unique, all while working within a unique set of circumstances,” said Sparks. “During their time at the helm, they personified the club’s true spirit, and have shown they have the potential to become successful first-team managers in the future.”
Trueman said: “It is obviously disappointing to have been stood down as manager, but I respect the board’s decision and am incredibly thankful for the opportunity.
“I have learnt an awful lot over the past six months and am incredibly grateful to have had the chance to lead a good group of people.
“I have been offered the opportunity to stay at the club and fully intend to do so. I hope to have my future decided as soon as possible, and am looking forward to continuing to work hard and develop.”
Sellars, who has held and lost his first managerial job before turning 30, commented: “I have loved every minute of the job and, despite my confidence in our ability to help take the club forward, I of course respect the decision made - following a disappointing end to the season - and my time as joint manager has taught me a lot.”
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